(1) Field of the Invention
This invention relates to a metal sheet having heat-bonded thereto a biaxially oriented polyester film or films, which sheet is used for cans and other containers. It also relates to a container, especially a side seamless container, including a closure, made of the polyester film-heat-bonded metal sheet.
(2) Description of the Prior Art
In order to impart corrosion resistance to metal sheets, the metal sheets have heretofore been coated with various synthetic resins. Typical synthetic resins used include thermosetting resins such as epoxy resins, unsaturated polyester resins, phenolic resins and thermosetting acrylic resins. The metal sheets also have been laminated with a film of thermoplastic resins such as polyesters, polyvinyl chloride and polyolefins. These thermoplastic resins exhibit generally a poor adhesion to the metal sheets and, therefore, there has been a need of using a suitable adhesive or primer. Among the thermoplastic resins, polyesters are advantageously used as a protective coating for cans and other containers in view of their adhesion, thermal resistance and hygienic qualities. Particularly, protective coatings of the following polyesters are popularly used: (i) polyethylene terephthalate (hereinafter abbreviated as "PET") and copolyesters predominantly comprised of ethylene terephthalate units, (ii) polybutylene terephthalate (hereinafter abbreviated as "PBT") and copolyesters predominantly comprised of butylene terephthalate units, and (iii) a polyblend of the polyester (i) or (ii) with an ethylene/vinyl acetate copolymer or polystyrene. A protective coating, which is a laminate of a biaxially oriented PET film adhered with a polyethylene film or another polyolefin film by using a adhesive, is also used in a manner such that the polyolefin film side of the laminate faces the metal sheet.
The above-mentioned conventional polyester protective coatings exhibit some of the following defects (i) through (v) when metal sheets laminated with the protective coatings are formed into containers particularly by means of drawing processing.
(i) Although the protective coatings have a satisfactory adhesion to a flank of the metal sheet prior to shaping processing, the protective coatings are liable to be cracked or be delaminated from the metal sheet either when the blank is subjected to severe shaping processing, such as folding and bending, double seaming with a can end, and drawing, or when the container is dropped.
(ii) Most polyester films having a satisfactory adhesion to the metal sheet are soft and sticky, and exhibit poor formability.
(iii) Most polyester protective coatings are not completely satisfactory in thermal resistance, and, when containers made therefrom are subjected to a retort treatment, the protective coverings are delaminated from the containers.
(iv) Because of poor corrosion resistance, the containers tend to be corroded after long term storage, leading to deterioration of the content.
(v) It is difficult to manufacture printed containers. In general, printed containers are manufactured by a process wherein a protective coating film is applied on one side of a metal sheet; the other side of the metal sheet is printed, and; then, the coated metal sheet is formed into containers. In the step of printing, particularly multicolor printing, the printed inks are baked usually at a temperature of at least 150.degree. C. for 30 to 60 minutes. Due to this baking, the protective coating has a tendency of thermally, deteriorating or crystallizing and, thus, the protective coating is liable to be cracked or crazed, or delaminated from the metal sheet in the step of forming into containers.
It is, therefore, a primary object of the present invention to provide a metal sheet having a polyester protective coating, which sheet overcomes the above-mentioned defects, namely, which possesses a good formability and is suitable for cans and other containers having excellent impact resistance, adaptability to retort treatment and corrosion resistance.
Other objects and advantages of the present invention will be apparent from the following description.